Material Selection Requirements
Identifying the appropriate material for your application is a big accomplishment. However, there is another choice that remains to be made. Keeping in mind the modern day emphasis on recycling, is the use of recycled and reground material an option for you? There is no universal answer to that question. Certain medical and optical applications may demand the use of virgin material only. In some applications, a percentage of in-house regrind with known history may be used. Yet in some others, a portion of post consumer recycled material of unknown origin may be tolerable. The two most important things to consider when mixing reground material with virgin are contamination and degradation of the regrind. Parts molded with contaminated material may cause appearance and delamination problems. Contamination sites have also been blamed for being the crack initiation points in many failed parts. Degraded regrind should be a matter of concern for all applications. Material degradation may be due to several heat histories, long residence times in the barrel and high moisture content during processing causing hydrolysis. Hydrolysis shortens molecular chains resulting in lesser molecular weight and loss of other physical properties. Melt indexing test is often used to estimate the degree of material degradation. Determing the amount of regrind to be used is quite subjective. The best way may be the trial and error method. Test the molded parts for performance as you increase the proportion of the regrind and arrive at a comfortable level. It is also important to ensure that the quality of the regrind is consistent good. |